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Dec 2004

Volume 42, Issue 9, pp. 516-562

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The Freezing of Streams and Ponds: A Simple—But Uncomfortable—Experiment

Craig F. Bohren

The Physics Teacher -- December 2004 -- Volume 42, Issue 9, pp. 522 | Cited 2 times

Online Publication Date: Nov 2004

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Within the span of a year, I twice faced the same question in slightly different forms. The first time it came from Tom Schlatter, who fields questions from readers of Weatherwise. He wrote to me that a reader had “asked why running water freezes more slowly than still water. The stock answer is friction, but I would like to do better than that.” I responded with quantitative arguments that (fluid) friction is irrelevant, and there the matter stood.
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01.50.My Demonstration experiments and apparatus
01.40.E- Science in school

A Generalized Wave Diagram for Moving Sources

Robert Alt and Sam Wiley

The Physics Teacher -- December 2004 -- Volume 42, Issue 9, pp. 526 | Cited 2 times

Online Publication Date: Nov 2004

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Many introductory physics texts1–5 accompany the discussion of the Doppler effect and the formation of shock waves with diagrams illustrating the effect of a source moving through an elastic medium. Typically these diagrams consist of a series of equally spaced dots, representing the location of the source at different times. These are surrounded by a series of successively smaller circles representing wave fronts (see Fig. 1). While such a diagram provides a clear illustration of the shock wave produced by a source moving at a speed greater than the wave speed, and also the resultant pattern when the source speed is less than the wave speed (the Doppler effect), the texts do not often show the details of the construction. As a result, the key connection between the relative distance traveled by the source and the distance traveled by the wave is not explicitly made. In this paper we describe an approach emphasizing this connection that we have found to be a useful classroom supplement to the usual text presentation. As shown in Fig. 2 and Fig. 3, the Doppler effect and the shock wave can be illustrated by diagrams generated by the construction that follows.
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01.40.-d Education
01.50.My Demonstration experiments and apparatus

Physics on the Bus—How About Physics on Your Bus?

Robert H. Romer

The Physics Teacher -- December 2004 -- Volume 42, Issue 9, pp. 528

Online Publication Date: Nov 2004

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Riders on the buses of UMass Transit (here in the “Five-College” area) and of the University of Georgia transit system have something other than the usual “Please don't eat on the bus” placards to read—simple physics questions in the overhead advertising racks, with appealing cats and dogs posing questions, giving partial answers (some flat-out wrong, some partially correct), ending up with “What do you think? … Visit our website.” One sample is given below; others can be viewed at http:∕∕www.amherst.edu∕∼physicsqanda. (More often than not, the dog gets it wrong; I'm a cat person.) This project was begun in collaboration with John King and was inspired by one of the many ideas described in his 2000 Oersted Medal address.
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01.40.-d Education

Kepler's Third Law Without a Calculator

Michael J. Ruiz

The Physics Teacher -- December 2004 -- Volume 42, Issue 9, pp. 530 | Cited 2 times

Online Publication Date: Nov 2004

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At the University of North Carolina at Asheville (UNC-A) we have used Kepler's third law in a liberal-arts conceptual astronomy course to help students sharpen their quantitative skills without using a calculator. Doing quantitative physics without a calculator represents one of the many ways we can study the physical world. Furthermore, it is fun.
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01.50.Kw Techniques of testing
01.40.J- Teacher training

Polymer Physics in an Introductory General Physics Course

Mark I. Liff

The Physics Teacher -- December 2004 -- Volume 42, Issue 9, pp. 536 | Cited 3 times

Online Publication Date: Nov 2004

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Do all solids expand upon heating? To most people's surprise, there is a class of rather common solids, namely rubbery elastic polymers, capable of contracting upon heating1,2 while staying in the same solid phase. This seems contradictory to common sense and the physical theories of thermal behavior of ordinary solids. The physical behavior of elastic polymers continues to amaze physics and chemistry students as well as many scientists, despite the fact that it was experimentally detected in natural rubbers two centuries ago. For the following 125 years this phenomenon remained unexplained. An explanation was finally found only in the 1930s when the new science of “polymer physics” emerged. The goal of this paper is to demonstrate that some elements of polymer physics can be useful in teaching introductory general physics, especially in discussing the thermal properties of solids and for introducing the concept of entropy. Initially, several simple demo∕lab experiments manifesting the extraordinary thermal properties of rubbers will be discussed. A brief description of the search for an explanation of the physics underlying this behavior will follow. The discussion will include the macromolecular hypothesis of Staudinger,3 the notion of a conformational state of a macromolecule, and the idea of statistical probabilities for the end-to-end macromolecular distances.1,2,4 The latter leads directly to an explanation of the emergence of the entropic force that is responsible for contraction upon heating. These notions are shown to be easily employable for introducing the idea of entropy to a beginner.
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01.50.Qb Laboratory course design, organization, and evaluation
01.40.G- Curricula and evaluation

A Physicist's View of the Automobile Engine

Bernard J. Feldman

The Physics Teacher -- December 2004 -- Volume 42, Issue 9, pp. 543 | Cited 2 times

Online Publication Date: Nov 2004

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Most first-semester university physics courses end with the chapter on the second law of thermodynamics, the Carnot cycle, and a brief introduction to the concept of entropy. My course and my final lecture are on the physics of the automobile engine. My students appreciate the application of the ideas of thermodynamics to an object that is an integral part of their lives, especially at a commuter school like mine. And the history of the automobile engine is a good lesson on the interaction between physics, engineering, economics, and environmental issues. It is a wonderful way to end the semester.
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01.40.Di Course design and evaluation
01.50.Qb Laboratory course design, organization, and evaluation
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